1. Field of the Invention
The present invention relates in general to four wheel drive motor vehicles of a type having a variable torque transfer which can control the driving force distribution ratio between front and rear wheels, and more particularly to a fluid pressure control device which controls the engaged condition of a variable torque clutch of the transfer by feeding the torque clutch with a controlled fluid pressure.
2. Description of the Prior Art
Japanese Patent First Provisional Publication 2-68225 shows a four wheel drive vehicle which is equipped with a transfer and a fluid pressure control device of the transfer. The vehicle disclosed by the publication is of a so-called "part-time four wheel drive vehicle" which is based on the front engine rear drive (FR) type. That is, in the four wheel drive mode of the vehicle, the driving force from the engine is distributed, through a driving force transmitting system including the transfer, to the front and rear wheels with a predetermined torque distribution ratio therebetween. The transfer is equipped with a variable torque clutch which can vary the torque distribution ratio by changing its engaged condition. That is, the engaged condition of the clutch is varied by a fluid pressure fed to a cylinder chamber of the clutch. The fluid pressure control device is connected to the cylinder chamber to feed the same with a pressure-controlled fluid. The fluid pressure control device has a fluid source from which the fluid is driven, by an electric motor or the like, to the cylinder chamber of the variable torque clutch through a check valve and a pressure control valve of proportional control type. The pressure control valve is controlled by an instruction current which is applied thereto from a control unit. That is, in accordance with the magnitude of the instruction current fed to the proportional solenoid of the pressure control valve, the outlet pressure of the pressure control valve, viz., the pressure (or clutch pressure) of the fluid fed to the variable torque clutch is varied. That is, the clutch pressure fed to the variable torque clutch is controlled in proportion to the magnitude of the instruction current from the control unit, and the engaged condition of the clutch is controlled by the controlled clutch pressure thereby controlling the torque distribution ratio between the front and rear wheels.
However, due to its inherent construction, the above-mentioned fluid pressure control device has the following drawbacks.
First, the clutch pressure fed to the transfer should be precisely controlled by only the pressure control valve. This inevitably induces the need of high costly pressure control valve.
Second, in the fluid pressure control device, there is employed an arrangement wherein to obtain the engaging force of the variable torque clutch which is proportional to the clutch pressure, the clutch pressure is continuously increased from a lower level by increasing the instruction current applied to the proportional solenoid. Thus, it takes time (viz., transient time) to change the drive mode from the two wheel drive mode to the four wheel drive mode. In the two wheel drive mode, the variable torque clutch is not in use, while, in the four wheel drive mode, the torque clutch is in use establishing a certain torque distribution between the front and rear wheels. In fact, the transient time is the period passed from the time when the clutch pressure is still low (two wheel drive mode) to the time when the clutch pressure has increased to a level sufficient for the four wheel drive mode.
In order to solve the above-mentioned drawbacks, various measures have been hitherto proposed, one of which is shown in. FIG. 6 of the accompanying drawings.
The fluid pressure control device shown in the drawing comprises a first fluid supply line 11 through which a pressurized fluid from a pressurized fluid source 10 flows. Designated by numerals 40 and 41 are a check valve and a fluid flow stabilizer respectively. The first fluid supply line 11 leads to an inlet portion of a pressure adjusting valve 12 by which the pressure of the fluid is adjusted to a predetermined level, that is, the line pressure level. An outlet port of the pressure adjusting valve 12 leads to a fluid tank 13, which parts constitute a drain system. The first fluid supply line 11 further leads to both an inlet port of a clutch pressure control valve 50 which can reduce the line pressure to a first lower level and an inlet port of a pilot valve 19 which can reduce the line pressure to a second lower level. The clutch pressure control valve 50 uses its outlet pressure as a pilot pressure of itself, and the outlet pressure of the valve 50 is led to a switch valve 51. The switch valve 51 has an outlet port led to a variable torque clutch "VTC", more specifically, to a cylinder chamber of the clutch "VTC". The pilot valve 19 sets its outlet pressure by using the same as a pilot pressure. The outlet pressure of the pilot valve 19 is led to a clutch pressure controlling electromagnetic valve 21. The electromagnetic valve 21 carries out an open and close operation at high speed to produce a continuously variable outlet pressure which is fed to the clutch pressure control valve 50 as a pilot pressure. With this, the outlet pressure of the clutch pressure control valve 50 is varied continuously that is, in a stepless manner.
The first fluid supply line 11 further leads to an inlet port of a switch valve controlling electromagnetic valve 24. The outlet pressure of the valve 24 is fed to the switch valve 51 as a pilot pressure. When electrically energized, the electromagnetic valve 24 takes ON (or fluid supply) position causing the switch valve 51 to assume a shut off position compressing its spool spring 51a. Under this condition, the associated motor vehicle takes the two wheel drive mode.
The fluid source 10 is powered by the engine of the associated motor vehicle. The pressure adjusting valve 12 uses the pressure of the fluid from the fluid source 10 as a pilot pressure. That is, when the fluid pressure from the fluid source 10 exceeds a predetermined level (viz., line pressure level), the pressure adjusting valve 12 opens and flows a part of the fluid to the drain system thereby keeping the fluid pressure at the predetermined line pressure level. The line pressure possessed by the fluid from the first fluid supply line 11 is reduced by a given degree by the clutch pressure control valve 50 and then fed to the switch valve 51 as a clutch pressure. The fluid pressure (viz., clutch pressure) produced by the clutch pressure control valve 50 is continuously varied or controlled by the continuously variable pressure provided by the clutch pressure controlling electromagnetic valve 21.
The operation of the switch valve 51 is controlled in ON/OFF manner by the switch valve controlling electromagnetic valve 24. That is, when the electromagnetic valve 24 is energized and thus takes the fluid supply position, the switch valve 51 takes OFF position, that is, the shut off position. Under this condition, the clutch pressure produced by the clutch pressure control valve 50 fails to reach the variable torque clutch "VTC" and thus the motor vehicle takes the two wheel drive mode. While, when the switch valve controlling electromagnetic valve 24 is deenergized and thus takes a fluid draining position, the switch valve 51 takes its ON position, as shown, permitting the clutch pressure to reach the variable torque clutch "VTC". Upon this, the motor vehicle takes the four wheel drive mode.
However, even the above-mentioned fluid pressure control device of FIG. 6 has the following drawbacks due to its inherent construction.
First, the pilot pressure continuously produced by the clutch pressure controlling electromagnetic valve 21 tends to have a marked pulsation due to the high speed reciprocating movement of the valve 21 by the instruction current applied thereto. The instruction current is in the form of a pulse signal. Furthermore, the pilot pressure produced by the valve 21 tends to vary widely due to high sensitivity of the valve 21 to the instruction current. These phenomena cause the variable torque clutch "VTC" to fail to exhibit a satisfied performance.
Second, in the fluid pressure control device, there is employed the arrangement wherein in order to switch the drive mode from the two wheel drive mode to the four wheel drive mode, the switch valve controlling electromagnetic valve 24 is forced to take the fluid draining position as shown. Thus, upon this switching, it tends to occur that the line pressure fed to the variable torque clutch "VTC" drops for a moment. Although this drawback may be solved by usage of a three-way valve, usage of such valve increases the production cost of the fluid pressure control device.